BTB walls with a single face seem to allow cars to pass through them in one direction even when you check the collide button and your material is double sided. I have resolved this problem using three sided walls and making an invisable texture and applying it to two faces. Collide must be checked in the material section of the walls menu.
Road materials must have a valid name the pulldown has rfactor material names they are not all valid for GTL and GTR2. If you use roada it will work for rfactor, gtl, gtr2, and gtr evo.